Recently, a demand for wireless data service is abruptly increasing. Evolution from wireless voice service to wireless data service requires a gradual increase in the wireless capacity. Such requirement makes wireless service providers and wireless equipment manufacturers try to improve the data transmission rate of wireless systems, giving them a motive to do active research.
Wireless channels experience several problems, such as path loss, shadowing, fading, noise, a limited bandwidth, a power limit of a terminal, and interference between users. Such limitations make the wireless channel have a form similar to a narrow pipe, hindering the fast flow of data, and also make it difficult to design an efficient bandwidth of wireless communication which provides high-speed data transmission. Other challenges in the design of a wireless system include resource allocation, mobility issues related to a rapidly changing physical channel, portability, and a design in which security and privacy are taken into consideration.
When a transmission channel experiences deep fading, if an additional version or replica of a transmitted signal is not received, it makes it difficult for a receiver to determine the transmitted signal. Resources, corresponding to the additional version or replica, are called diversity. Diversity is one of the most important factors contributing to reliable transmission through wireless channels. If such diversity is employed, the capacity or reliability of data transmission can be maximized. A system, implementing diversity using multiple transmission antennas and multiple reception antennas, is called as a Multiple Input Multiple Output (MIMO) system.
In an MIMO system, schemes for implementing diversity include Space Frequency Block Code (SFBC), Space Time Block Code (STBC), Cyclic Delay Diversity (CDD), Frequency Switched Transmit Diversity (FSTD), Time Switched Transmit Diversity (TSTD), Precoding Vector Switching (PVS), and Spatial Multiplexing (SM) schemes.
Meanwhile, one of systems taken into consideration, from among systems subsequent to the third generation systems, is an Orthogonal Frequency Division Multiplexing (OFDM) system which can attenuate an inter-symbol interference effect through a low complexity. In the OFDM system, serial input data is converted into an N number of parallel data, and the parallel data is carried on an N number of orthogonal subcarriers and then transmitted. The subcarriers maintain orthogonality in the frequency domain. An Orthogonal Frequency Division Multiple Access (OFDMA) scheme refers to a multi-access method of realizing multi-access by independently providing some of available subcarriers to each of users in a system using OFDM as a modulation scheme.
However, one of the major problems of the OFDM/OFDMA systems is that a Peak to Average Power Ratio (PAPR) may be very high. The PAPR problem means that the peak amplitude of a transmission signal is very greater than the mean amplitude. The PAPR problem results from the fact that an OFDM symbol is the overlapping of an N number of sinusoidal signals on different subcarriers. In particular, the PAPR is related to the capacity of the battery and is problematic in a terminal sensitive to power consumption. In order to reduce power consumption, it is necessary to lower the PAPR.
One of systems proposed to lower the PAPR is Single Carrier-Frequency Division Multiple Access (SC-FDMA). SC-FDMA is of a form in which the Frequency Division Multiple Access (FDMA) method is grafted onto the Single Carrier-Frequency Division Equalization (SC-FDE) method. The SC-FDMA scheme is similar to the OFDMA scheme in that data is modulated and demodulated in the time and frequency domains using a Discrete Fourier Transform (DFT), but is advantageous for reducing transmission power because the PAPR of a transmission signal is low. In particular, in relation to the use of the battery, the SC-FDMA scheme may be said to be advantageous in uplink communication from a terminal, sensitive to transmission power, to a base station. An important point when a terminal sends data to a base station is a wide coverage where power can be concentrated although the bandwidth of transmitted data is not great. An SC-FDMA system makes small a change in the signal and thus has a wider coverage than other systems when the same power amplifier is used because.
In order to apply an MIMO transmission scheme for sending multiple codewords to an SC-FDMA system, not only a single carrier characteristic, but also a low PAPR has to be guaranteed. If a DFT-spread signal is switched in a resource element unit and mapped to frequency resources, a low PAPR cannot be guaranteed after IFFT. Accordingly, there is a need for a method of sending multiple codewords, capable of performing MIMO transmission while maintaining a low PAPR.